Atmosphere
The standard atmosphere (symbol: atm) is an international reference pressure defined as 101325 and formerly used as a unit of . BS 350:2004 Conversion Factors for Units For practical purposes it has been replaced by the which is 105 Pa. The difference of about 1% is not significant for many applications, and is within the error range of common pressure gauges. History In 1954 the 10th Conférence Générale des Poids et Mesures ( ) adopted standard atmosphere for general use and affirmed its definition of being precisely equal to 1,013,250 s per (101 325 ).BIPM Definition of the standard atmosphere This value was intended to represent the mean atmospheric pressure at mean sea level at the latitude of , and as a practical matter, truly reflects the mean sea level pressure for many of the industrialized nations (those with latitudes similar to Paris). In , the original definition of “Standard Temperatureand Pressure” ( ) was a reference temperature of 0 (273.15 ) and pressure of 101.325 (1 atm). However, in 1982, the (IUPAC) recommended that for the purposes of specifying the physical properties of substances, “''the standard pressure''” should be defined as precisely 100 kPa (exactly 1 bar).IUPAC.org, Gold Book, Standard Pressure Pressure units and equivalencies A pressure of 1 atm can also be stated as: :≡1.013 25 :≡ 101325 (Pa) :≡ 1013.25 s (mbar, also mb) :≡ 760 B :≈ 760.001 mm-Hg, 0 °C, subject to revision as more precise measurements of mercury’s density become available C :≈ 29.9213 in-Hg, 0 °C, subject to revision as more precise measurements of mercury’s density become available C :≈ 1.033 227 452 799 886 /cm² :≈ 1.033 227 452 799 886 :≈ 1033.227 452 799 886 cm–H2O, 4 °C A :≈ 406.782 461 732 2385 in–H2O, 4 °C A :≈ 14.695 948 775 5134 (psi) :≈ 2116.216 623 673 94 pounds-force per square foot (psf) :Notes: :A This is the customarily-accepted value for cm–H2O, 4 °C. It is precisely the product of 1 kg-force per square centimeter (one technical atmosphere) times 1.013 25 (bar/atmosphere) divided by 0.980 665 (one gram-force). It is not accepted practice to define the value for water column based on a true physical realization of water (which would be 99.997 495% of this value because the true maximum density of is 0.999 974 95 kg/l at 3.984 °C). Also, this “physical realization” would still ignore the 8.285 cm–H2O reduction that would actually occur in a true physical realization due to the vapor pressure over water at 3.984 °C. :B Torr and mm-Hg, 0°C are often taken to be identical. For most practical purposes (to 5 significant digits), they are interchangeable. :C value of 13.595 078(5) g/ml assumed for the density of Hg at 0 °C Other applications s and others use the word atmosphere and "atm" in relation to pressures that are relative to mean atmospheric pressure at sea level (1.013 bar). For example, a partial pressure of oxygen is calibrated typically using air at sea level, so is expressed in units of atm. The old European unit (at) is roughly equal to the gauge pressure under 10 m of water; 1 at = 98066.5 Pa. See also * * *Bar References Category:Units of pressure or stress Category:Atmosphere Category:Chemical engineering Category:Environmental engineering Category:Mechanical engineering Category:Physical chemistry Category:Standards Category:Thermodynamics